1. Field of the Invention
Exemplary aspects of the present invention generally relate to a developing device employing a stirring mechanism for a two-component developer, and an image forming apparatus including the developing device.
2. Description of the Background
A related-art image forming apparatus, such as a copier, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor); an optical scanning device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; the electrostatic latent image is developed with a developer (e.g., a toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
In such an image forming apparatus, either a one-component developer consisting essentially of a toner (e.g., magnetic toner and non-magnetic toner) or a two-component developer including a toner and a carrier which carries the toner is used for development.
The toner, when agitated and mixed into the carrier, is frictionally charged so as to be electrostatically attracted to the electrostatic latent image formed on the photoconductor. Thus, the toner is consumed during development whereas the carrier is not.
A typical developing device, which holds the developer, generally includes a developing sleeve, configured to form a magnetic brush of the developer on a surface thereof and to supply the developer to the electrostatic latent image formed on the photoconductor, and an agitator sleeve, configured to supply agitated developer to the developing sleeve. Developer in which the toner has been consumed in the development of the electrostatic latent image formed on the photoconductor is collected and returned to the developing device.
Consumption of the toner included in the developer causes a decrease in image density, and therefore fresh toner needs to be supplied to the developer. The fresh toner may be supplied from above a conveyance screw including a screw auger serving as the agitator sleeve, or from an edge of a rotation shaft of the conveyance screw.
The fresh toner is supplied to the developer based on developer density detected by a toner density sensor or the like, in amounts controlled by controlling a rotation of a supply member configured to supply the fresh toner stored in a toner supply unit. When the fresh toner is poured into the developer in a developer tank, the fresh toner and a carrier are agitated by the conveyance screw provided in the vicinity of the supply member so that the developer is frictionally charged. As a result, the developer having a predetermined or desired charge is supplied to the developing sleeve.
Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2001-188408 discloses a developing device including a screw auger configured to agitate a developer to frictionally charge the developer as described above. JP-A 11-143196 discloses a developing device in which an agitating unit provided separately from a developing unit at a position where a developer is circulated agitates collected developer and supplied toner so that the resultant developer is frictionally charged. Japanese Patent No. (hereinafter referred to as JP) 3734096 discloses a developing device in which an agitating unit provided separately from a developing unit includes a screw auger configured to move a developer upward.
In general, the supplied toner is dispersed throughout the developer, and the developer is frictionally charged by being agitated by rotation of the screw auger for a short time until the developer thus prepared is conveyed to the developing sleeve. Consequently, the degree of mixing depends in part on the amounts supplied. Thus, when a larger amount of toner is supplied to the developer, the toner may not be dispersed sufficiently in the developer in the brief time allotted for agitation, and consequently, the toner may not be charged sufficiently when discharged from the developer tank. As a result, weakly charged toner could reach the developing sleeve, fouling of a surface of the photoconductor and scattering over peripheral components, thereby degrading image quality.
When the screw auger described above is used, only that developer which contacts the screw auger itself is agitated, as is the case when using a stirring paddle. Consequently, the supplied toner may not be sufficiently dispersed in the developer and reliably charged in the developer tank.
One possible method for solving the above-described problem is to increase a rotation speed of the screw auger to cause the toner to contact the carrier more frequently so that a predetermined or desired charge is reliably applied to the developer. However, the screw auger driving system may be damaged due to the increased transfer resistance to the developer when the developer is agitated. Moreover, the toner may be damaged due to increased force of impact on the developer and heat caused by increased friction, increasing stress on the developer.